Digital Signal Processing Reference
In-Depth Information
AGC
LPF
A/D
I
RF LNA
BPF
I/Q LO
LPF
A/D
Q
Fig. 21
Principal structure of direct-conversion radio receiver
the image signal band is the desired signal itself (at negative center-frequency), and
the I/Q imbalances cause self-image interference. Other practical implementation
problems, stemming from direct RF-baseband downconversion, are LO leakage and
DC offsets, or in general second order intermodulation (IM2), which create spurious
signal energy and interference on top of the desired signal. We will discuss these
4.4.3
Low-IF Receiver
In the basic low-IF receiver, in order to reduce the effects of LO leakage and
DC offsets, the desired signal is I/Q or quadrature downconverted to a low but
non-zero IF. Thus the basic structure is similar to previous direct-conversion block-
diagram but the complex I/Q signal after I/Q downconversion is located at low
intermediate frequency. As an example, intermediate frequencies in the order of
one or two channel bandwidths have been proposed and considered. Selectivity
can be implemented with special complex analog bandpass filters, centered at low
IF, or then with more wideband lowpass filter after which the final selectivity and
downconversion from IF to baseband is carried out digitally after A/D interface.
Notice that since the image signal in RF-IF downconversion comes now again from
another channel/band with a (possibly) very high power level, the use of a non-zero
IF reintroduces the image signal problem to big extent and the practical 20-40 dB
image attenuation of analog I/Q downconversion can easily be insufficient.
In a “per-channel” downconverting low-IF receiver, the image signal originates
from one of the nearby (adjacent) channels. Though the image problem is in this
case partly alleviated by the system specifications, which usually limit the power
difference of the nearby channels to 10
40 dB attenuation pro-
vided by a practical analog front-end is clearly inadequate for most communication
waveforms. In a multichannel scenario, which is especially interesting, e.g., on
the base station side of cellular systems, several channels are downconverted as
...
25 dB, the 20
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